Thanks ctcoker, that is a big help. I have keyed in this data and got the following results for perihelion periods and alignments. For a minute there I thought EM Bary was the architect of Covent Garden!
Using the formula which Hornblower kindly gave me a few years ago, C = 1/(1/A - 1/B), gives the following periods in years for perihelion conjunctions
Degrees per Year Perihelion Period
Jupiter 0.00181992 197,810.94
Earth 0.003179526 113,224.42
Venus 0.000567965 633,842.10
Unfortunately for my supposition that the eccentricity cycles might be a function of alignment of perihelions, these numbers are nothing like the 400, 125 and 95 ky periods I hoped to see. So I am still stumped as to why Jupiter and Venus would produce terrestrial orbital eccentricity cycles of this length. I will simply take Hornblower's latest post as an indicator that the calculations are far too complicated for me to worry about, although a simple explanation of this basic process would have added to the Muller and McDonald book.
Jupiter Earth 264,782.51
Earth Venus 137,848.58
Jupiter Venus 287,550.32
To illustrate what I thought would be the answer, but it seems is not, I made the attached diagram showing the elliptical alignments. Any further enlightenment appreciated.